An LED printhead is part of a non-impact printer which employs an array of light emitting diodes (commonly referred to herein as LEDs) for exposing a photoreactive surface. The resulting pattern impressed upon the photoreactive surface is then transferred onto paper, or like material, in a way well known in the art.
In a typical LED printer, a row, or two closely spaced or staggered rows, of minute LEDs are positioned near an elongated lens array so that their images are focused onto the surface to be illuminated. The LEDs are driven by constant current integrated circuit power supplies which are switched on or off to create the desired image on the photoreactive surface.
In such a printer, all of the LEDs must produce substantially similar light output power (LOP) to produce a uniform print quality. However, left uncompensated, the light output of LEDs can vary greatly. Non-uniformities are introduced to the LOP in a variety of ways.
One cause of non-uniformities in LED output power is the variation in LED efficiency (light output as a function of current) due to the materials used in the LED wafers and fabrication of the LEDs themselves. Another cause of non-uniformities is variations in the drive current supplied by integrated power supplies due to similar concerns. These non-uniformities are inherent in the light output of the LEDs and they exist regardless of controlling other operating parameters such as temperature.
These non-uniformities are typically eliminated by individually calibrating the exposure time of each LED, thereby ensuring that the light output power for each LED exposure is approximately uniform. This is accomplished by measuring the LOP of each printhead LED, calculating the exposure time for each LED needed to produce a uniform LOP, and storing the calculated values in memory on the printer itself. Thereafter, when the printer is in use, these pre-determined values are used to control the exposure time of the LEDs.
This "one time" calibration of LED exposure power is often insufficient where precision LOP is required Temporal instability in the LED light output produces non-uniformities that must be eliminated on a periodic basis. One source of temporal instability is the long-term degradation of the LED light output power as the total LED on-time increases. This degradation is caused by the increase in the concentration and/or the cross section of non-radiative recombination centers near the LED junction. The concentration and type of crystalline defects associated with this recombination depends on many factors related to the fabrication of the LEDs and the magnitude of the degradation varies from LED to LED.
A second temporal instability is caused by the variation of LED light output power due to the heating and cooling of the entire printhead in use and to ambient temperature changes. For example, under normal operation, the printhead as a whole may see up to a 30.degree. C. temperature rise which will cause a 27% loss in LOP.
A third source of temporal instability is the variation in LOP from LED to LED over short periods of time due to spatially varying power inputs into the LED printhead. Such non-uniformities are caused by the local heating of each LED as it and its neighbor LEDs are turned on and off. While the long-term temporal instabilities occur on the order of hundreds of hours, the short term spatially varying instabilities occur on the order of seconds. All of these non-uniformities must be corrected in a high precision and high speed printer.
U.S. Pat. No. 4,780,731, to Creutzmann discloses an electrophotographic printer that incorporates a "one time" calibration of LED exposure power on an LED-to-LED basis. The electrographic printer also includes a photoresponsive element positioned for acquiring the LOP transmitted onto the recording medium. To be precise, the photodetector element is positioned outside of the lens and is thus susceptible to toner build-up on its photoreactive surface. Also, the photodetector element is swivelably secured to the printhead and must be pivoted into the path of the focused light emitted from the lens each time the LOP is measured, thus adding to the mechanical complexity of the printhead. The LOP measured by the photodetector element is used periodically, in conjunction with the other operating parameters, to uniformly define a common operating parameter, such as LED drive current, for all of the LEDs. The assignee of the Creutzmann patent, Siemens Aktieageseilschaft, has published data specifications for a product implementing the subject matter of the Creutzmann patent which further discloses that several LED drive currents may be defined for each of a plurality of groups of LEDs. The printer thus compensates for the long-term temporal instabilities in the printhead which are uniform to all LEDs, or groups of LEDs.
However, as previously described, high precision printers are susceptible to other temporal instabilities that vary from LED to LED. It is desirable, therefore, to provide an LOP monitor and feedback system for an LED printhead that intermittently compensates for non-uniformities in LOP on an LED-to-LED basis, or at least in groups of LEDs.